Excitation hierarchy of the spin-1 large-D system NiCl₂-4SC(NH₂)₂

NiCl₂-4SC(NH₂)₂ (known as DTN) is an S = 1 chain system with the easy-plane anisotropy dominating over the exchange interaction (so-called large-D system) and a new candidate for studying the fieldinduced Bose-Einstein condensation of magnons. The excitation spectrum of DTN has been investigated by means of tunable-frequency ESR technique in fields up to 25 T. Based on analysis of the magnon excitation spectrum, a revised set of spin-Hamiltonian parameters was obtained. These values were used to calculate the AFM phase boundary, low-temperature magnetization and the frequency-field dependence of two-magnon bound-state excitations, predicted by theory and observed in DTN for the first time. Excellent quantitative agreement with experimental data was obtained. Published in Phys. Rev. Lett. 98, 047205 (2007).